The present invention relates generally to power conversion and conditioning and, more particularly, to a PWM rectifier with a variable switching frequency.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as low frequency three-phase AC current.
Despite being distributable efficiently, low frequency AC current is sometimes not suitable for end use in consuming facilities. Thus, prior to end use power delivered by a utility has to be converted to a useable form. To this end a typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end-useable form (e.g., three phase relatively high frequency AC voltage). A controller controls the inverter in a manner calculated to provide voltage waveforms required by the consuming facility.
Motors and their associated loads are one type of common inductive load employed at many consuming facilities. While the present invention is applicable to different load types, to simplify this explanation, an exemplary motor with an associated load will be assumed. To drive a motor an inverter includes a plurality of switches that can be controlled to link and delink the positive and negative DC buses to motor supply lines. The linking-delinking sequence causes voltage pulses on the motor supply lines that together define alternating voltage waveforms. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. The magnetic field induces (hence the nomenclature “induction motor”) a field in motor rotor windings. The rotor field is attracted to the rotating stator field and hence the rotor rotates within the stator core.
A pulse width modulation (PWM) rectifier is one type of rectifier employed in a high performance adjustable speed drive (ASD) where regeneration or high quality input current is required. As AC drives proliferate, equipment system specifications limiting the amount of harmonic current injected into the utility grid are becoming more common and thus solicit cost effective harmonic mitigation solutions. System specifications are often written so measured total harmonic distortion at the Point of Common Coupling (PCC) complies with the maximum low voltage total harmonic distortion levels (THDV) and system classification of IEEE 519. The PCC is usually at the power metering point where other customers connect to the common line voltage but may also be within a plant where linear and non-linear loads are connected. Diode rectifiers typically encounter difficulty in attempting to met the harmonic distortion constraints.
Typical PWM rectifiers are better able to meet the harmonic distortion requirements, but are more costly than conventional diode rectifiers. This cost differential has typically precluded the use of PWM rectifiers in medium performance applications. As compared to a diode rectifier, a PWM rectifier requires an additional input LCL filter on the source side, main inductors on the conversion side, and switching devices used to implement the PWM technique. Two parameters associated with the performance of PWM rectifiers are ripple current in the main inductors and switching losses. Significant ripple current results in a higher required rating for the main inductors, thereby increasing their cost. Various techniques have been employed to improve the performance of PWM circuits. Space vector PWM control (SVPWM) and discrete PWM (DPWM) are two techniques commonly used PWM techniques. In a DPWM system, only two phases are modulated at any given time, thereby reducing switching losses to 50% as compared to SVPWM for a unity power factor condition. However, both techniques show significant ripple current, necessitating a larger, more expensive main inductor.
It would be desirable for a PWM rectifier to exhibit lower ripple currents, thereby reducing the cost of the rectifier and widening its range of applications.